Relational database systems store tables of data which are typically linked together by relationships that simplify the storage of data and make queries of the data more efficient. Structured Query Language (SQL) is a standardized language for creating and operating on relational databases.
Relational database systems can have the capability to save a compiled execution plan for an SQL query and to re-use the saved plan for subsequent execution of the same query. Storing the compiled execution plan saves the resource cost (e.g. processing time) of having to repeatedly parse and optimize frequently executed queries. A relational database system typically includes an optimizer that plans the execution of SQL queries. The optimizer's chosen plan for a given query can change as data characteristics change or the optimizer's software itself changes.